How can you catch clap if you and your partner swear blind no one has cheated?? can it be passed from passing water infection back and forth?

Only by sex: Gonorrhea (clap) is only transmitted sexually. One of you caught it from another partner. But gonorrhea can be carried for a few months, especially by women and rarely men, so if your relationship is fairly new, it doesn't necessarily mean he cheated. A urinary tract (water) infection can cause similar symptoms, so maybe UTI was confused with gonorrhea. Most important, you both need treatment.

Gonorrhea: this STD is spread through sexual contact. Women who have it may have no symptoms at all, so may spread it unknowingly. Men usually always have symptoms. At some point someone in the relationship was exposed to this disease. It is important for you both to be treated as there are potentially serious long term effects if not treated.